In addition to those components which serve to simulate the joint function, joint endoprostheses have parts for anchoring them to the bone. If different anchoring parts are kept available for one prosthesis, the anchoring part chosen in each case is connected to an associated joint component via a connection device. For this purpose, cone connections have proven themselves which consist of a cone stub and of a cone sleeve which are designed with the same slight cone angle, for example of 5xc2x0, and which generally stick sufficiently firmly in each other as soon as they have been joined together under the effect of an impact. Unless considerable forces arise acting in the direction of separation of the connection, further securing means are not necessary. This applies, for example, when fastening hip-joint heads to the associated stem parts (U.S. Pat. No. 5,336,268). If substantial releasing forces have to be taken into account, the cone connection is provided with a securing screw (EP-A 474015). For example, a grub screw can be included in the cone stub, the conical tip of which screw is screwed into engagement with a bore, eccentric thereto, of the cone sleeve (EP-A 474015, FIG. 3).
It is also known (U.S. Pat. No. 5,405,403) for a cone connection to be provided with an anti-rotation arrangement in order to prevent relative turning between the cone surfaces, as this could lead to corrosion.
A securing screw not only prevents a translatory movement of the coupled parts in the axial direction, but also prevents a rotational movement. Thus, a securing screw traditionally also performs the functions of an anti-rotation arrangement.
The invention has taken recognition of the fact that considerable advantages can be achieved by combining a securing screw with an anti-rotation arrangement, if the securing screw interacts with a bore, eccentric thereto, via a conical tip. When such a securing screw is used without an anti-rotation arrangement, as in the prior art, it is not possible to guarantee that the conical tip of the securing screw will interact precisely with that point of the bore edge which lies nearest to the thicker end of the cone connection during assembly. It is only then, in fact, that exact anti-rotation by the securing screw is obtained. However, if the contact point lies slightly to the side of this ideal point of interaction, microrotation movements are possible. The combination according to the invention thus ensures, however, that a certain contact point between the conical tip, the securing screw and the associated bore is fixed. This not only prevents the said micro movements, but also prevents the securing screw from becoming worn, from twisting and possibly from wholly or partially losing its securing function as a result of relative movements with respect to the bore edge which interacts with it.
This is of particular importance when the securing screw is provided with a predetermined break point (EP-A 915686=WO 9804215). The anti-rotation arrangement thus has the effect that the securing screw can fulfil its function in the desired manner.
To provide this anti-rotation arrangement, projections and recesses are expediently formed on the two coupling parts and interact with each other via axially parallel flanks. For example, at least one stub can project in an axially parallel manner from the edge of the cone sleeve and engage in a correspondingly shaped groove which is provided at the base of the cone stub. Conversely, it is also possible for the edge of the cone sleeve to be provided with at least one indentation into which a transverse pin provided on the cone stub engages.